Tachykinin receptors are the targets of a family of structurally related peptides which include substance P (SP), neurokinin A (NKA) and neurokinin B (NKB), named collectively “tachykinins”. Tachykinins are synthesized in the central nervous system (CNS) and peripheral tissues, where they exert a variety of biological activities. Three tachykinin receptors are known which are named neurokinin-1 (NK-1), neurokinin-2 (NK-2) and neurokinin-3 (NK-3) receptors. Tachykinin receptors belong to the rhodopsin-like seven membrane G-protein coupled receptors. SP has the highest affinity and is believed to be the endogenous ligand of NK-1, NKA for NK-2 receptor and NKB for NK-3 receptor, although cross-reactivity amongst these ligands does exist. The NK-1, NK-2 and NK-3 receptors have been identified in different species. NK-1 and NK-2 receptors are expressed in a wide variety of peripheral tissues and NK-1 receptors are also expressed in the CNS; whereas NK-3 receptors are primarily expressed in the CNS.
The neurokinin receptors mediate a variety of tachykinin-stimulated biological effects that include transmission of excitatory neuronal signals in the CNS and periphery (e.g. pain), modulation of smooth muscle contractile activity, modulation of immune and inflammatory responses, induction of hypotensive effects via dilatation of the peripheral vasculature and stimulation of endocrine and exocrine gland secretions.
In the CNS, the NK-3 receptor is expressed in regions including the medial prefrontal cortex, the hippocampus, the thalamus and the amygdala. Moreover, NK-3 receptors are expressed on dopaminergic neurons. Activation of NK-3 receptors has been shown to modulate dopamine, acetylcholine and serotonin release suggesting a therapeutic utility for NK-3 receptor modulators for the treatment of a variety of disorders including psychotic disorders, anxiety, depression, schizophrenia as well as obesity, pain or inflammation (Giardina et al., Exp. Opinion Ther. Patents, 2000, 10(6), 939-960; Current Opinion in Investigational Drugs, 2001, 2(7), 950-956 and Dawson and Smith, Current Pharmaceutical Design, 2010, 16, 344-357).
Schizophrenia is classified into subgroups. The paranoid type is characterized by delusions and hallucinations and absence of thought disorder, disorganized behavior, and affective flattening. In the disorganized type, which is also named ‘hebephrenic schizophrenia’ in the International Classification of Diseases (ICD), thought disorder and flat affect are present together. In the catatonic type, prominent psychomotor disturbances are evident, and symptoms may include catatonic stupor and waxy flexibility. In the undifferentiated type, psychotic symptoms are present but the criteria for paranoid, disorganized, or catatonic types have not been met. The symptoms of schizophrenia normally manifest themselves in three broad categories, i.e. positive, negative and cognitive symptoms. Positive symptoms are those, which represent an “excess” of normal experiences, such as hallucinations and delusions. Negative symptoms are those where the patient suffers from a lack of normal experiences, such as anhedonia and lack of social interaction. The cognitive symptoms relate to cognitive impairment in schizophrenics, such as a lack of sustained attention and deficits in decision making. The current antipsychotic drugs (APDs) are fairly successful in treating the positive symptoms but fare less well for the negative and cognitive symptoms. Contrary to that, NK-3 antagonists have been shown clinically to improve on both positive and negative symptoms in schizophrenics (Meltzer et al, Am. J. Psychiatry, 2004, 161, 975-984) and ameliorate cognitive behavior of schizophrenics (Curr. Opion. Invest. Drug, 2005, 6, 717-721).
In rat, morphological studies provide evidence for putative interactions between NKB neurons and the hypothalamic reproductive axis (Krajewski et al, J. Comp. Neurol., 2005, 489(3), 372-386). In arcuate nucleus neurons, NKB expression co-localizes with estrogen receptor α and dynorphin, implicated in progesterone feedback to Gonadotropin Releasing Hormone (GnRH) secretion (Burke et al., J. Comp. Neurol., 2006, 498(5), 712-726; Goodman et al., Endocrinology, 2004, 145(6), 2959-2967). Moreover, NK-3 receptor is highly expressed in the hypothalamic arcuate nucleus in neurons which are involved in the regulation of GnRH release.
WO 00/43008 discloses a method of suppressing gonadotropin and/or androgen production with specific NK-3 receptor antagonists. More particularly, the WO 00/43008 application relates to lowering luteinizing hormone (LH) blood level by administering an NK-3 receptor antagonist. Concurrently or alternatively with gonadotropin suppression, WO 00/43008 also relates to suppression of androgen production with NK-3 receptor antagonists. Recently it has been postulated that NKB acts autosynaptically on kisspeptin neurons in the arcuate nucleus to synchronize and shape the pulsatile secretion of kisspeptin and drive the release of GnRH from fibers in the median eminence (Navarro et al., J. of Neuroscience, 2009, 23(38), 11859-11866). All these observations suggest a therapeutic utility for NK-3 receptor modulators for sex hormone-dependent diseases.
NK-3 receptors are also found in the human myenteric and submucosal plexus of the sigmoid colon as well as in the gastric fundus (Dass et. al., Gastroenterol., 2002, 122 (Suppl 1), Abstract M1033) with particular expression noted on myenteric intrinsic primary afferent neurons (IPANs) (Lomax and Furness, Cell Tissue Res, 2000, 302, 59-3). Intense stimulation of IPANs changes patterns of intestinal motility and intestinal sensitivity. Electrophysiology experiments have shown that activation of the NK-3 receptor changes the voltage threshold of action potentials in IPANs and promotes the generation of long-lasting plateau potentials (Copel et al., J Physiol, 2009, 587, 1461-1479) that may sensitize these neurons to mechanical and chemical stimuli leading to effects on gut motility and secretion. Similarly, Irritable Bowel Syndrome (IBS) is characterized by patient hypersensitivity to mechanical and chemical stimuli. Thus, NK-3 antagonists have been tested in preclinical models of IBS where they have been shown to be effective to reduce nociceptive behavior caused by colo-rectal distension (Fioramonti et al., Neurogastroenterol Motil, 2003, 15, 363-369; Shafton et al., Neurogastroenterol Motil, 2004, 16, 223-231) and, on this basis, NK-3 antagonists have been advanced into clinical development for the treatment of IBS (Houghton et al., Neurogastroenterol Motil, 2007, 19, 732-743; Dukes et al., Gastroenterol, 2007, 132, A60).
Non-peptide antagonists have been developed for each of the tachykinin receptors. Some of them have been described as dual modulators able to modulate both NK-2 and NK-3 receptors (WO 06/120478). However, known non-peptide NK-3 receptor antagonists suffer from a number of drawbacks, notably poor safety profile and limited CNS penetrability that may limit the success of these compounds in clinical development.
On this basis, new potent and selective antagonists of NK-3 receptor may be of therapeutic value for the preparation of drugs useful in the treatment and/or prevention of CNS and peripheral diseases or disorders in which NKB and the NK-3 receptors are involved.
Target potency alone, which may be demonstrated by competitive binding data, is not sufficient for drug development. Rather, efficacy in vivo is contingent upon achieving a relevant “free” drug concentration relative to the target potency at the physiological site of action. Drug molecules typically bind reversibly to proteins and lipids in plasma. The “free” fraction refers to the drug concentration that is unbound and therefore available to engage the biological target and elicit pharmacological activity. This free fraction is commonly determined using plasma protein binding (PPB) assays. The free drug fraction is relevant to not only achieving the desired pharmacological activity, but also potentially undesirable activities including rapid hepatic metabolism (leading to high first-pass clearance and thereby poor oral bioavailability) as well as possible off-target activities that can lead to safety concerns (for example, inhibition of hERG ion channel activity, a widely accepted marker of cardiovascular toxicity).
The invention thus encompasses compounds of general Formula I, their pharmaceutically acceptable solvates as well as methods of use of such compounds or compositions comprising such compounds as antagonists to the NK-3 receptor. Compounds of Formula I are N-acyl-(3-substituted)-(8-methyl)-5,6-dihydro-[1,2,4]triazolo[4,3-a]pyrazines. The compounds of the invention are generally disclosed in international patent application WO2011/121137 but none is specifically exemplified therein. On another hand, unsubstituted and thus non-chiral 5,6,7,8-tetrahydro[1,2,4]triazolo[4,3-a]pyrazines have been disclosed in WO2010/125102 as modulators of an unrelated target, namely P2X7.